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I purchased a 1972 413 from a 1973 motorhome, it has had very little use, and extremely clean internally. I am curious as to what the compression ratio might be, any ideas would be helpful, it has a pretty good dish in the top of the piston. It is still the stock bore and stock pistons. I have replaced the heads, they had holes in the ends ( steam holes) and an extremely large 4 armed water pump. I have replaced the heads with 906 castings.

You can not use the original manifold with those 906 heads, it used water in the heat riser passages not exhaust. Before you go too far make sure you have the later style flange on the crank. The old style indexed on the outside of the flange rather than the lip in the center. Could also be a gear driven camshaft which rotates the opposite direction from normal.

I think I read something awhile back saying that the truck engines had around 7.5.1 compression. Not exactly a neck snapper. If I was you , I would get some flattop pistons in there to boost the output. I pulled one apart and it had the gear drive that drove the cam the opposite direction. The pushrods were longer than a regular 440, and it had some sort of spacer for the distributor.

Thanks for the help, yes the pushrods and head bolts are longer, I was lucky, the engine when I purchased it came with the transmission. I have replaced the head bolts and push rods with the correct size for the 906 heads. I have replaced the oil pan, since the motor home pan had a front sump. I am curious if the transmission will work for a Challenger (what I purchased the engine for in the first place) it seems the correct size, I think it is a 727, but not sure. All your help is greatly appreciated.

i also dragged one of the old 413's home this past weekend. origin unknown? has 4-10-68 cast on the block, stock bearings had 68 stamped on them, which leads me to believe the block is a 68. crank uses the lip to locate flex plate (like slant6/ or small block) motor has chain driven cam, flat top pistons, and closed chamber heads(200 castings). i think it may have the hipo exhaust manifolds, due to their funky curvy shape? no water heat for the heat riser.
factory cast iron 4bbl intake and thermo bog. looking for a set of slugs UNDER $400!?! (cheapest i have found so far) had a 71 car 727 hooked up to it. got the whole chunk for $50 (motor&tranny)
planning to do a close to stock rebuild with max overbore and enough cam to help the closed chamber heads flow a bit. gona drop the thing in a 74 duster and use it for a strip car. cheap (sortoff) and fast, not so much like the slant i have built up! (best time of 16.08 in a 4 dr 78 aspen 3.21 8-1/4 suregrip/904) good daily driver car.
anyone know if the 4.250 pistons from a 383 have the same comp height? (that will make it a 426
thanks all.....chad

If the origanal heads were closed chamber and you replaced them with open chamber 906's you will have very low compression.
Also I don't think anybody makes an aftermarket piston for these engines. You might be able to find stock low compression replacement pistons.

without plugging the #s into the comp calculator, my guess is this is a stock 9-10:1 motor. flat top pistons with a -.015 deck height , steel shim head gaskets, and closed chambers, 7.5:1?
i have found a stock replacement piston for the engine, sealed power hyperuetectics, .030 over, flat top, bout $50 a piece. the pistons in the motor now measure 4.180 (413 bore size) hmmmm
chad